Low impedance implantable extension for a neurological electrical stimulator

ABSTRACT

A medical device known as an implantable neurostimulation system is configured for implanting in humans to deliver a therapeutic electrical stimulation to tissue to treat a variety of medical conditions such as pain, movement disorders, pelvic floor disorders, and many other conditions. The implantable neurostimulation has a housing, a power supply carried in the housing, stimulation electronics coupled to the battery and coupled to a neurostimulator connector block, a stimulation lead, and a lead extension. The lead extension is electrically coupleable between the neurostimulation connector block and the stimulation lead. The extension conductor is composed of an outer surface and an inner core. The outer surface has an outer impedance and the inner core has a core impedance that is substantially lower than the outer impedance. Many embodiments of the low impedance lead extension are possible.

RELATED APPLICATION

This application is a divisional of prior application Ser. No.09/893,851, filed Jun. 28,2001, now U.S. Pat. No. 6,671,544.

BACKGROUND OF THE INVENTION

This disclosure relates to a medical device and more particularly toimplantable neurological electrical stimulators and implantableelectrical stimulation leads.

The medical device industry produces a wide variety of electronic andmechanical devices for treating patient medical conditions such aspacemakers, defibrillators, neurostimulators, and therapeutic substancedelivery pumps. Medical devices can be configured to be surgicallyimplanted or connected externally to the patient receiving treatment.Clinicians use medical devices alone or in combination with therapeuticsubstance therapies and surgery to treat patient medical conditions. Forsome medical conditions, medical devices provide the best and sometimesthe only therapy to restore an individual to a more healthful conditionand a fuller life. One type of medical device is an implantableneurological stimulation system that can be used to treat conditionssuch as pain, movement disorders, pelvic floor disorders, gastroparesis,and a wide variety of other medical conditions. The neurostimulationsystem typically includes a neurostimulator, a stimulation lead, and anextension such as shown in Medtronic, Inc. brochure “ImplantableNeurostimulation System” (1998).

Previous extensions are typically formed using a solid conductor formedfrom a material that is a reasonably good compromise between themechanical properties required to form electrical connections and theconductive properties required to efficiently conduct the stimulationsignal from the neurostimulator to the stimulation lead. The compromiseof material used in a solid conductor results in higher impedance thanis desired. The extension's higher impedance than desired can result inincreased power consumption and decreased battery life. An example of anextension that uses a solid conductor is shown in Medtronic, Inc.brochure “Model 7495 Extension Kit for Stimulation of the Brain, SpinalCord, or Peripheral Nerves” (2000). An example of a low impedance leadused for cardiac pacing and defibrillation typically having only one ortwo conductors is shown in U.S. Pat. No. 5,330,521 “Low ResistanceImplantable Electrical Leads” by Cohen (Jul. 19, 1994).

For the foregoing reasons, there is a need for a low impedance extensionthat decreases power consumption and has many other improvements.

SUMMARY OF THE INVENTION

A low impedance extension for an implantable neurological electricalstimulator embodiment reduces energy consumption and has many otherimprovements. The low impedance extension has a conductor composed of anouter surface and an inner core. The outer surface has an outerimpedance and the inner core has a core impedance that is substantiallylower than the outer impedance. The low impedance extension has anextension proximal end, an extension distal end and an extension body.The extension proximal end has at least one proximal contact coupleableto an implantable neurostimulator connector block and an extensiondistal end having at least one extension distal contact coupleable tothe lead proximal contact. The extension conductor is contained in theextension body to electrically connect the extension distal contact withthe extension proximal contact. Many embodiments of the low impedanceextension are possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the environment of an implantable therapeutic substancedelivery device embodiment;

FIG. 2 shows a neurostimulation system embodiment;

FIG. 3 a shows a low impedance extension embodiment;

FIG. 3 b shows a cross section of the low impedance extension bodymulti-lumen embodiment;

FIG. 3 c shows a longitudinal cross section of an extension conductormulti-lumen embodiment;

FIG. 3 d shows a cross-section of an extension conductor withoutinsulator embodiment;

FIG. 3 e shows a cross-section of an extension conductor with insulatorembodiment;

FIG. 3 f shows a cross-section of a stranded extension conductorembodiment;

FIG. 3 g shows a cross-section of a stranded extension conductor withinsulator embodiment;

FIG. 4 a shows a multi-filar coil single extension conductor embodiment;

FIG. 4 b shows a multi-filar coil multi-extension conductor embodiment;

FIG. 5 a shows a multi-conductor coil extension conductor embodiment;

FIG. 5 b shows a multi-lumen linear extension conductor embodiment; and,

FIG. 5 c shows a multi-conductor linear extension conductor embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the environment of an implantable medical device known asan implantable neurological electrical stimulation system 20. Theneurological stimulation system 20 can be used for a wide variety oftherapies such as pain, movement disorders, pelvic floor disorders,gastroparesis, and many other medical conditions. Implantation of theneurological stimulation system 20 typically begins with eitherpercutaneous or surgical implantation of a stimulation lead 22 typicallywhile the patient is under local anesthetic. Once the stimulation lead22 has been implanted and positioned, the stimulation lead's distal end24 is typically anchored into position to minimize movement of thestimulation lead 22 after implantation. The stimulation lead's proximalend 26 is connected to a lead extension 28 that has been tunneled to thelocation where a neurological electrical stimulator, also known as aneurostimulator 30 is to be implanted. The lead extension 28 isconnected to the neurostimulator 30 and the neurostimulator 30 istypically implanted into a subcutaneous pocket at a site selected afterconsidering clinician and patient preferences. The neurostimulator 30can be programmed to generate a stimulation signal with a voltageamplitude typically in the range from about 0 V to about 10.5 V; a pulsewidth typically in the range from about 30 μS to about 450 μS; and apulse rate typically in the range from about 2 Hz to about 180 Hz.

FIG. 2 shows an implantable neurostimulation system 20 having a lowimpedance extension 28 comprising an implantable neurostimulator 30, astimulation lead 22, and a lead extension 28. The implantableneurostimulator 30 has a housing 32, a power supply 34 such as a batterycarried in the housing 32, and stimulation electronics 36 coupled to thepower supply 34 and coupled to a connector block 38. The stimulationlead 22 has a lead proximal end 26, a lead distal end 24, and a leadbody 40. The lead proximal end 26 has at least one electrical contact 42and the lead distal end 24 has at least one stimulation electrode 44.There is at least one lead conductor 48 contained in the lead body 40that electrically connects the lead electrical contact 42 to thestimulation electrode 44.

FIGS. 3 a-3 g show various low impedance extension 28 embodiments. Thelead extension 28 has an extension proximal end 50, an extension distalend 52, and an extension body 54. The extension proximal end 50 has atleast one proximal contact 56 coupleable to the implantableneurostimulator connector block 38. The extension distal end 52 has atleast one extension distal contact 58 coupleable to the lead proximalelectrical contact 42. There is at least one extension conductor 60contained in at least one lumen 62 of the extension body 54 electricallyconnecting the extension distal contact 58 with the extension proximalcontact 56. In some embodiments such as shown in FIGS. 3 f and 3 g, morethan one wire strand 64 can be used to form a single extension conductor60. Functionally, the lead extension 28 serves as a means for leadextension coupleable between the implantable neurostimulator connectorblock 38 and the lead proximal contact 42 to electrically connect theneurostimulator 30 to the stimulation lead 22.

The extension conductor 60 is composed of an outer surface 66 and aninner core 68. The outer surface 66 has an outer impedance and the innercore 68 has a core impedance that is substantially lower than the outerimpedance. The resistivity ratio of the outer surface 66 to the innercore 68 is at least about 2:1. The low impedance conductor 60 resistanceis in the range from about 0.05 to 0.3 ohms per centimeter of extensionlength. Resistance can be calculated using the formula$R = \frac{\rho\quad L}{A}$where R is resistance in ohms, ρ is resistivity in ohm-centimeters, L isconductor length in centimeters, and A is conductor cross-section areain centimeters squared. Functionally, the extension conductor 60 servesas a means for extension of electrical connectivity that has an outersurface 66 selected for mechanical properties and an inner core 68selected for electrical conductivity with an impedance substantiallyless than the outer surface 66. The extension conductor 60 can beconstructed of drawn filled tubing. The extension conductor 60 can havean outer insulator 70 surrounding the outer surface 66 of the extensionconductor 60 (FIG. 3 e). The outer insulator 70 can be composed of awide variety of materials with electrically insulating properties suchas fluoropolymer, polyurethane, silicone, polyimide and the like.

The outer surface 66 provides the mechanical properties of the extensionconductor 60 such as corrosion resistance and fatigue life. The outersurface 66 can be composed of a wide variety of materials that providethe desired mechanical characteristics such as nickel, cobalt, chrome,molybdenum alloy, stainless steel, and the like. The outer surface 66has a resistivity of greater than about 25 micro ohm-centimeter.Functionally, the outer surface 66 serves as a means for outer surfaceselected for the mechanical properties of flexibility and electricalconnection formation.

The inner core 68 provides the low impedance properties of the extensionconductor 60. The inner core 68 can be composed of a wide variety ofmaterials having low impedance properties such as silver, silver alloy,gold, copper, platinum, iridium, tantalum, aluminum, and the like. Theinner core 68 has a resistivity of less than about 12.5 microohm-centimeter. Functionally, the inner core 68 serves as a means forconductivity having a core impedance that is substantially lower thanthe outer impedance.

FIGS. 4 a-4 b show multi-filar coil extension conductor 60 embodiments.The extension conductor 60 can have a wide variety of configurations inthe extension body 54 to meet the needs of the extension 28. At leastone extension conductor 60 can be configured as at least one coil 72 inat least one lumen 62 of the extension body 54. In multi-filar coilembodiments where the coil filars 74 are uninsulated (FIG. 4 a), themulti-filar coil acts as a single conductor 60. Extension conductors 60such as shown in FIGS. 3 d and 3 f can be used to form the multi-filarcoil embodiment shown in FIG. 4 a. The extension conductor 60 can alsohave an outer insulator 70. When the coil filars 74 are insulated (FIG.4 b), the each filar 74 can act as an individual conductor 60 and istermed a multi-conductor coil. Extension conductors 60 such as shown inFIGS. 3 e and 3 g can be used to form the multi-filar coil embodimentshown in FIG. 4 b. The coil can have at least one filar 74 and someembodiments can have two, four, eight, sixteen, or more filars 74. Therecan be at least one extension conductor 60 configured as at least onelinear wire in at least one lumen 62. In the linear wire embodiments(FIGS. 5 b-5 c), each filar 74 is an extension conductor 60. The linearwire can have more than one strand, and the linear wire can have anouter insulator 70. Depending on the extension body 54 embodiment, theconductor 60 may or may not be insulated. In each case, at least oneconductor 60 connects at least one extension proximal contact 56 to atleast one extension distal contact 58.

FIGS. 5 a-5 c show various alternate extension body embodiments. Theextension body 54 can have at least one lumen 62, and the extension body54 can be configured with a separate lumen 62 for each filar 74. Someextension bodies 54 embodiments can be configured with between two andsixteen lumens 62. FIG. 5 a shows a cross-sectional view of amulti-conductor coil 78 extension body 54 embodiment. Themulti-conductor coil 78 is contained in a single lumen 62 tubingextension body 54. Each filar 74 of the coil 78 is insulated providingat least as many conductors 60 as filars 74. FIG. 5 b shows across-sectional view of a linear wire 80 extension body 54. Eachmulti-strand 64 filar resides in a lumen 62 of an extension body 54 thathas more than one lumen 62. The physical separation provided by theextension body 54 isolates the wire conductors 60 from each other.Additionally, the linear wire 80 may be electrically insulated toprovide redundant electrical insulation.

FIG. 5 c shows a cross-sectional view of a linear wire 80 in a singlelumen 62 extension body 54. Each linear wire 80 is individuallyinsulated to provide electrical isolation between conductors 60 andcontained within the extension body 54. In all embodiments (FIGS. 5 a-5c), at least one conductor 60 electrically connects at least oneextension proximal contact 56 to at least one extension distal contact58.

Thus, embodiments of the low impedance extension 28 for a neurologicalstimulator 30 are disclosed to increase energy efficiency and providemany other improvements. One skilled in the art will appreciate that thepresent invention can be practiced with embodiments other than thosedisclosed. The disclosed embodiments are presented for purposes ofillustration and not limitation, and the present invention is limitedonly by the claims that follow.

1. An low impedance conductor for a low impedance extension, comprising:an outer surface selected for mechanical properties of flexibility andelectrical connections having an outer impedance; and, an inner coreselected for conductivity having a core impedance that is substantiallylower than the outer impedance; wherein the inner core an outer surfacehave a composite resistance in the range from about 0.05 to about 0.3ohms per centimeter.
 2. The low impedance conductor as in claim 1,wherein the extension conductor is constructed of drawn filled tubing.3. The low impedance conductor as in claim 1, wherein the extensionconductor is constructed of more than one wire strand.
 4. The impedanceconductor as in claim 1, wherein the extension conductor has an outerinsulator surrounding the outer surface of the extension conductor.